1. Field of the Invention
The present invention relates to thermoplastic boat hulls and methods of construction of such thermoplastic boat hulls generally, and to the construction of a trimaran sailboat in particular.
2. Discussion of the Prior Art
Boat hulls have been constructed for centuries out of wood and other natural materials with large quantities of time expended in order to interconnect the pieces of wood in a generally water tight manner. Within the last thirty years, the use of polyester resin and fiberglass reinforcement has lead to the ability to create unusual boat hull configurations and has been widely accepted in the boat building industry.
Unfortunately, the construction of a fiberglass boat hull requires not only a mold to be made, but also a rather labor-intensive hand laying up of layers of fiberglass cloth, roving reinforcement, etc. While the use of more exotic materials such as Kevlar.TM. and carbon fiber have resulted in extremely lightweight hulls, a significant portion of the cost of any modern fiberglass boat is the labor cost of constructing the hull. The layers of reinforcement materials have to be laid into the mold, the resin added and then sealed or otherwise compressed to obtain the strongest yet lightest resultant structure.
The problem of high labor cost in the construction of boat hulls has been addressed in the kayak hull construction industry in which boat hulls have been made utilizing the "roto-molding" process with the hull material being made of a thermoplastic material, in a preferred embodiment polyethylene. A mold is created whose inner cavity corresponds to the desired outer dimensions of the complete vessel. The desired quantity of thermoplastic polyethylene pellets is added to the mold and the temperature of the mold brought up to above the melting point for the polyethylene. The mold is at the same time rotated about the hull's longitudinal axis and at least rocked back and forth (if not rotated) about the boat's transverse axis. In this way, the entire inner portion of the cavity is coated with the melted polyethylene material.
By varying the local temperature of different portions of the mold, variations in coating thickness can be achieved. When sufficient rotations have occurred to create the desired thickness of melted polyethylene at the various portions of the mold, the mold is cooled (to solidify the polyethylene) and opened to remove the vessel.
For use in small kayaks or similar boat hulls, the polyethylene plastic is a relatively inexpensive structural material. However, it is not rigid and is a relative low modulous material, not nearly as stiff as fiberglass. Consequently, rigidity must be achieved either with the hull shape and/or skin thickness. Unfortunately, thick skins of polyethylene are also heavy relative to some other boat building materials.
The use of rolo-molding eliminates the need for labor intensive hand lay up of the various layers of fiberglass reinforcement in a conventional fiberglass hull. Consequently, due to the inexpensive nature of the polyethylene plastic and the fact that the rolo-molding is a more-or-less automatic operation, the cost for making a kayak-type boat hull is relatively inexpensive. However, conventional kayak designs are not sufficiently strong and stable to withstand the stress of serious sailing, and normally there is no provision for being able to power such a craft by sail.
Consequently, there is a need for a boat hull shape which can be easily coated in the mold cavity by rolo-molding, which has the necessary distribution of polyethylene (in terms of skin thickness) so as to be sufficiently rigid to withstand the stress of sailing while at the same time not being overly thick so as to be overly heavy and which can be easily propelled through the water either by human power, sail power or small motor power.
It would be desirable to provide a boat hull which is roto-moldable and can be used in conjunction with a sail for propulsion. Again, to withstand the stresses of the sail, some attention to the ability to rolo-mold an even thickness with appropriate increases in thickness in areas related to the support of the mast and cross beams would be necessary. Furthermore, one problem with existing sail powered craft, especially relatively small ones, is that when sailing in a chop, a fair amount of tore and aft rocking or "hobby horsing" motion exists. Quite obviously, this can be uncomfortable to the sailor but of more importance is the fact that this causes the mast and the sail to be pitched tore and aft changing the relative wind on the sail in terms of velocity and angle. The result of this movement is that the sail acts much more inefficiently than it would without the hobby horsing motion.